Most Recent Weekly Report: 1 November-7 November 2006

The number of daily earthquakes beneath Atka increased in July and remained elevated into November. Episodes of volcanic tremor that first occurred in September increased in number, strength, and duration in the past several weeks. On 28 October, residents of Atka observed steam emissions to many hundreds of meters above the summit. On 6 November, the AVO raised the Aviation Level of Concern Color Code to Yellow and the Volcanic Alert Level to Advisory due to the high seismicity and steam emissions.

Most Recent Bulletin Report: November 2006 (BGVN 31:11)

Minor plumes bearing steam and ash throughout November-December 2006

Korovin, the most frequently active cone in the Atka volcanic center, has been relatively quiet since 23 February [2005], when it emitted minor though abruptly discharged steam and ash (BGVN 31:02). At that time, an initial ash burst rose to an altitude of ~ 2.4 km and was followed by several smaller ash-and-steam bursts, but no ashfall was reported in Atka village (figure 2). There were no reports of accompanying volcanic odors, earthquakes, or larger volcanic explosions. While there was minor steaming during the period 25 February-4 March 2005, seismicity at Korovin remained only slightly above background levels. During that time frame, the Alaska Volcano Observatory (AVO) raised the concern color code at Korovin from green to yellow but reduced it to green on 8 March.

Figure 2. Sketch map of the central Aleutian Islands including Atka Island and the city of Atka (population 27, according to 1999 US Census estimate). Courtesy of USGS/AVO.

An increase in seismicity during July 2006 represented a transition from prior low activity, meanwhile volcanic activity remained mild during that time (figure 3).

Figure 3. The Korovin cone (lower left) in the Atka volcanic center was puffing steam to ~ 300 m above the summit on 30 July 2006 when seen from an Alaska Airlines jet. The Kliuchef cone (upper right, with two snow-clad craters) is one of multiple satellite cones. Photo credit to Cyrus Read; image courtesy of AVO/USGS.

This report also covers further minor events during September through December. During September and October, episodes of volcanic tremor increased in number, strength, and duration. On 28 October, residents of Atka village observed and photographed steam emissions to several hundred meters above the volcano.

For the duration of November and December, seismic levels remained above background levels. In late November satellite information showed a light dusting of ash on the E flank of the main crater along with several plumes and/or their shadows visible along the N side of the crater. Satellite radar images indicated uplift of the volcano; the area of uplift was consistent with locations of earthquake activity and the effects were interpreted as the result of magma injection. Cloud cover permitted only erratic satellite observation during November and December. On 11 and 21 December 2006, Atka residents again witnessed steam plumes, on the latter date possibly containing ash.

Index of Weekly Reports

Weekly Reports

1 November-7 November 2006

The number of daily earthquakes beneath Atka increased in July and remained elevated into November. Episodes of volcanic tremor that first occurred in September increased in number, strength, and duration in the past several weeks. On 28 October, residents of Atka observed steam emissions to many hundreds of meters above the summit. On 6 November, the AVO raised the Aviation Level of Concern Color Code to Yellow and the Volcanic Alert Level to Advisory due to the high seismicity and steam emissions.

AVO decreased the Concern Color Code at Korovin volcano in the Atka volcanic center from Yellow to Green (the lowest level) on 8 March. After raising the Concern Color Code on 22 February in response to increased seismicity, the rate of micro-earthquakes stabilized and then declined. During 1-8 March, seismicity was near background levels and no unusual activity was seen on satellite imagery or by observers.

Seismicity at Korovin volcano in the Atka volcanic center remained slightly above background levels during 24 February to 3 March. Clouds obscured satellite views of the volcano, and AVO received no reports of activity. There were no indications that an eruption was imminent. The volcano remained at Concern Color Code Yellow.

AVO reported that the Concern Color Code at Korovin volcano in the Atka volcanic center was raised from Green to Yellow on 22 February due to an increase in seismicity at the volcano. Distinct seismic signals indicating unrest were recorded on 4 days between 17 and 22 January, with a sustained 11-minute-long seismic signal on 22 February. After 22 February, seismicity decreased and distinct seismic signals like those recorded earlier were not detected. Clouds obscured satellite views of the volcano after 22 February. A pilot report on the 22nd indicated that the summit area was obscured by clouds, and there were no signs of ashfall on the flanks and no steam plume above the volcano. No obvious signs of activity were seen on 23 February by observers in the village of Atka. AVO received no reports of volcanic activity and there were no indications that an eruption was imminent.

AVO did not receive reports of activity at Korovin volcano in the Atka volcanic center after the original report of ash-and-steam emissions on 24 February. During 25 February to 4 March, cloud cover prohibited satellite views of the volcano and no unusual seismicity was registered. Korovin is not monitored by a standard AVO seismic network. Therefore, AVO did not assign a Concern Color Code to the volcano during the report period.

On 24 February AVO raised the Concern Color Code at Korovin volcano of the Atka volcanic center from Green to Yellow after receiving a report that ash and steam were emitted from Korovin on 23 February around 1900. According to residents of Atka village near the volcano, the initial ash burst rose to a height of ~ 2.4 km a.s.l. and drifted E. It was followed by several smaller ash-and-steam bursts. No ashfall was reported in Atka village, nor were there reports of accompanying volcanic odors, earthquakes, or larger volcanic explosions. Satellite images of the volcano did not clearly show the presence of ash or any thermal anomalies. On the morning of 24 February the volcano was steaming. AVO warned that low-level steam-and-ash emissions may continue and could pose a hazard to people and low- to medium-flying aircraft in the vicinity of the volcano. They further warned that if the eruption continues and begins to intensify, light ash fall could occur on parts of Atka Island, including the village of Atka.

Bulletin Reports

All information contained in these reports is preliminary and subject to change.

05/1986 (SEAN 11:05)Steam emission follows magnitude 7.7 earthquake

[On 6 May], James Dickson noted a 500-m vertical steam plume over the SE part of the summit [of Korovin]. On 23 May at about 1730, Aleutian Air pilot Thomas Madsen noted a 600-m steam plume containing traces of ash rising above the rim of a 240-m-deep crater in the SE part of the summit area. A shallow magnitude 7.7 earthquake centered roughly 100 km [SSW] of Korovin at [51.52°N, 174.78°W] occurred 7 May [at 2247 GMT].

John Reeder and Kirk Swanson climbed [Korovin] in July 1983, observing only minor steaming from deep within the crater, rising to only 60 m above its floor.

On 18 March at 1954 a NOAA 10 satellite image showed three distinct plumes, each 95 km long, drifting ENE. The estimated vent locations were: 52.38°N, 174.15°W (Korovin's summit); 52.31°N, 174.24°W; and 52.29°N, 174.21°W (5.5 km WSW and 6.5 km SW of Mt. Kliuchef, a cone on Korovin's S flank). More accurate locations will be determined by USGS Anchorage. Just before midnight on the same day US Navy pilot Jeffrey Sullivan observed a southward-drifting ash plume rising to at least 3,000 m altitude, lit by an orange "flame" from Korovin. Smaller orange flickering "flames" from two other vents at lower elevation were visible. Two of the vents were ~2 km apart and the third vent was ~10 km NE of the other two vents.

Earlier in the day (at 1300) Julie Dirks and other Atka residents noticed sulfur smells (~18 km from the volcano). Although the weather was clear Dirks did not notice any "unusual" eruptive activity. On 19 March a SIGMET notice was issued to warn pilots of volcanic ash 185 km on either side of a line from 52°N, 175°W to 54°N, 172°W. The warning remained in effect until 0930. Pilots reported that the ash cloud reached 3,600 m.

On 10 December AVO reported the following. "On 4 December, following a M 5.1 earthquake at 1836 beneath the . . . island of Atka, residents of Atka village reported "rumbling" from the cluster of volcanoes that forms the N part of the island. At the same time, residents observed increased steaming from a fumarolic area on the flank of Kliuchef volcano and noted a distinct sulphur smell. A check of satellite imagery did not confirm any change in the surface state of the volcanoes. Late this week, the weather had deteriorated and no visual observations of the volcanoes were possible. . . ."

Observers in the village of Atka on Atka Island in the central Aleutians reported a very strong sulfur smell on 1 May, and to a lesser extent on 4 May when they observed a small plume-like cloud over Kliuchef. Fumarolic areas exist on or near both Korovin and Kliuchef volcanoes, N of the village. Korovin was active most recently in 1987, and Kliuchef has had Holocene activity. Satellite images did not reveal any plume-like clouds associated with the island, however, a "hot-spot" possibly of fumarolic areas, was observed. A similar sulfur smell was reported by residents in December 1993, at about the same time they felt rumbling from a M 5.1 earthquake (BGVN 18:11).

On 29 June, Japan Airlines reported volcanic ash erupting from Atka. In addition, GEOS-9 satellite images showed a possible small ash cloud in the immediate vicinity of Atka. In early May 1995 residents of Atka village observed a small plume-like cloud over Kliuchef and reported a strong sulfur smell (BGVN 20:05).

Atka is the largest volcanic center in central Aleutians and consists of a central shield and caldera ringed by seven or eight satellitic volcanoes. Some of them have been active in historical time; Korovin was active most recently in 1987, and Kliuchef has had Holocene activity.

06/1998 (BGVN 23:06)Low eruption plume seen in June, dusting of ash in Atka

At about 1000 on 30 June, the Alaska Volcano Observatory (AVO) received a report of an eruption at Korovin from an observer in the village of Atka, near the volcano. The crew of a Coast Guard C-130 airplane confirmed that a low-level eruption plume had risen to almost 5 km above sea level by 1030, and late in the afternoon a pilot reported the plume at 9 km. The low-level ash-and-steam plume was not visible on satellite imagery due to meteorological clouds. Local winds at the time were light and to the SSW. A dusting of ash was reported in Atka. Poor weather on 1 July prohibited both direct and satellite observations.

Korovin volcano is located on the north end of Atka Island in the central Aleutians (figure 1), 538 km W of Dutch Harbor. It is 21 km N of the village of Atka, which has a population of about 100. The last reported eruption was in March 1987. AVO does not maintain seismic monitoring equipment on Atka Island.

The Alaska Volcano Observatory (AVO) received a report that a sudden minor steam-and-ash eruption occurred at Korovin around 1900 on 23 February [2005]. Korovin is an active volcano of the larger Atka volcanic center (or complex) in the central Aleutian islands, ~ 184 km E of Adak island and ~ 600 km W of Unimak island, and 1,760 km SW of Anchorage. According to residents of Atka village near Korovin, the initial ash burst rose to an altitude of ~ 2.4 km and drifted E. It was followed by several smaller ash-and-steam bursts.

No ashfall was reported in Atka village, nor were there reports of accompanying volcanic odors, earthquakes, or larger volcanic explosions. Satellite images of the volcano did not clearly show the presence of ash or any thermal anomalies. But, on the morning of 24 February [2005] (the morning after the eruption) the volcano was still steaming with some vigor. During the period 25 February-4 March [2005], cloud cover prohibited satellite views of the volcano, and no unusual seismicity registered.

Although Korovin was not monitored by a standard AVO seismic network, Atka village hosts a seismic station operated by the Alaska Earthquake Information Center. [No activity was reported following the 23 February 2005 eruption until January 2006]. Distinct seismic signals indicating unrest were recorded on 17, 18, 21, and 22 January 2006. The later day brought an unusually sustained, 11-minute-long signal.

After 22 February, seismicity decreased and distinct seismic signals like those recorded earlier were not detected. An 8 March report noted that Korovin's rate of occurrence of micro-earthquakes had stabilized and then declined, and "it has been close to background levels for the past week."

For some days after 22 February, clouds obscured satellite views of the volcano. However, on the 22nd an aviator reported that, although the summit area was obscured by clouds, no signs of ashfall on the flanks or any steam plume was seen. On 23 February, observers in the village of Atka noted the lack of obvious signs of activity. Prior to the ash-and-steam eruption on the 23rd, AVO received no short-term reports of precursory volcanism to indicate an imminent eruption.

Seismicity at Korovin remained slightly above background levels during 24 February to 3 March 2006. Clouds continued to mask satellite views of the volcano, and AVO received no further reports of activity.

Korovin, the most frequently active cone in the Atka volcanic center, has been relatively quiet since 23 February [2005], when it emitted minor though abruptly discharged steam and ash (BGVN 31:02). At that time, an initial ash burst rose to an altitude of ~ 2.4 km and was followed by several smaller ash-and-steam bursts, but no ashfall was reported in Atka village (figure 2). There were no reports of accompanying volcanic odors, earthquakes, or larger volcanic explosions. While there was minor steaming during the period 25 February-4 March 2005, seismicity at Korovin remained only slightly above background levels. During that time frame, the Alaska Volcano Observatory (AVO) raised the concern color code at Korovin from green to yellow but reduced it to green on 8 March.

Figure 2. Sketch map of the central Aleutian Islands including Atka Island and the city of Atka (population 27, according to 1999 US Census estimate). Courtesy of USGS/AVO.

An increase in seismicity during July 2006 represented a transition from prior low activity, meanwhile volcanic activity remained mild during that time (figure 3).

Figure 3. The Korovin cone (lower left) in the Atka volcanic center was puffing steam to ~ 300 m above the summit on 30 July 2006 when seen from an Alaska Airlines jet. The Kliuchef cone (upper right, with two snow-clad craters) is one of multiple satellite cones. Photo credit to Cyrus Read; image courtesy of AVO/USGS.

This report also covers further minor events during September through December. During September and October, episodes of volcanic tremor increased in number, strength, and duration. On 28 October, residents of Atka village observed and photographed steam emissions to several hundred meters above the volcano.

For the duration of November and December, seismic levels remained above background levels. In late November satellite information showed a light dusting of ash on the E flank of the main crater along with several plumes and/or their shadows visible along the N side of the crater. Satellite radar images indicated uplift of the volcano; the area of uplift was consistent with locations of earthquake activity and the effects were interpreted as the result of magma injection. Cloud cover permitted only erratic satellite observation during November and December. On 11 and 21 December 2006, Atka residents again witnessed steam plumes, on the latter date possibly containing ash.

The largest volcanic center in the central Aleutians, Atka consists of a central shield and Pleistocene caldera with several post-caldera volcanoes. A major dacitic explosive eruption accompanied formation of the caldera about 500,000 to 300,000 years ago. The most prominent of the post-caldera stratovolcanoes are Kliuchef and Sarichef, both of which may have been active in historical time. Sarichef has a symmetrical profile, but the less eroded Kliuchef is the source of most if not all historical eruptions. Kliuchef may have been active on occasion simultaneously with Korovin volcano to the north. Hot springs and fumaroles are located on the flanks of Mount Kliuchef and in a glacial valley SW of Kliuchef.

This compilation of synonyms and subsidiary features may not be comprehensive. Features are organized into four major categories: Cones, Craters, Domes, and Thermal Features. Synonyms of features appear indented below the primary name. In some cases additional feature type, elevation, or location details are provided.

Cones

Feature Name

Feature Type

Elevation

Latitude

Longitude

Kliuchef
Kliuchevskoy

Stratovolcano

1451 m

52° 19' 54" N

174° 8' 12" W

Sarichef
Sarutschew
Sarytchev

Stratovolcano

1056 m

52° 19' 0" N

174° 2' 48" W

Thermal

Feature Name

Feature Type

Elevation

Latitude

Longitude

Milky River

Thermal

52° 19' 12" N

174° 8' 50" W

An aerial view from the WNW shows a young satellitic vent on the northeast shoulder of Kluichef volcano. Clouds touch the tip of Sarichef volcano in the center distance. Kliuchef lies within a Pleistocene Atka caldera and is the most recently active of the post-caldera stratovolcanoes at Atka.

The following references have all been used during the compilation of data for this volcano, it is not a comprehensive bibliography. Discussion of another volcano or eruption (sometimes far from the one that is the subject of the manuscript) may produce a citation that is not at all apparent from the title.

WOVOdat is a database of volcanic unrest; instrumentally and visually recorded changes in seismicity, ground deformation, gas emission, and other parameters from their normal baselines. It is sponsored by the World Organization of Volcano Observatories (WOVO) and presently hosted at the Earth Observatory of Singapore.

EarthChem develops and maintains databases, software, and services that support the preservation, discovery, access and analysis of geochemical data, and facilitate their integration with the broad array of other available earth science parameters. EarthChem is operated by a joint team of disciplinary scientists, data scientists, data managers and information technology developers who are part of the NSF-funded data facility Integrated Earth Data Applications (IEDA). IEDA is a collaborative effort of EarthChem and the Marine Geoscience Data System (MGDS).